Re: [Qemu-devel] [PATCH v2 3/5] vvfat: Replace bdrv_{read, write}() with bdrv_{pread, pwrite}()

[Eric Blake]

On 5/6/19 11:47 AM, Vladimir Sementsov-Ogievskiy wrote:
> 01.05.2019 21:13, Alberto Garcia wrote:
>> There's only a couple of bdrv_read() and bdrv_write() calls left in
>> the vvfat code, and they can be trivially replaced with the byte-based
>> bdrv_pread() and bdrv_pwrite().
>>
>> Signed-off-by: Alberto Garcia <address@hidden>
>> ---
>>   block/vvfat.c | 12 +++++++-----
>>   1 file changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/block/vvfat.c b/block/vvfat.c
>> index 5f66787890..253cc716dd 100644
>> --- a/block/vvfat.c
>> +++ b/block/vvfat.c
>> @@ -1494,8 +1494,8 @@ static int vvfat_read(BlockDriverState *bs, int64_t 
>> sector_num,
>>                   DLOG(fprintf(stderr, "sectors %" PRId64 "+%" PRId64
>>                                " allocated\n", sector_num,
>>                                n >> BDRV_SECTOR_BITS));
>> -                if (bdrv_read(s->qcow, sector_num, buf + i * 0x200,
>> -                              n >> BDRV_SECTOR_BITS)) {
>> +                if (bdrv_pread(s->qcow, sector_num * BDRV_SECTOR_SIZE,
>> +                               buf + i * 0x200, n) < 0) {
> 
> Shouldn't we use QEMU_ALIGN_DOWN(n, BDRV_SECTOR_SIZE) ?

No, n should already be aligned, which makes align_down a no-op.

> Could bdrv_is_allocated give unaligned n?
> 

Yes, bdrv_is_allocated can return unaligned n in some situations; I had
a patch that didn't make 4.0 that would add bdrv_block_status_aligned
for cases where we need to guarantee that different alignment of a
backing chain doesn't bleed through to the specified alignment of the
current layer. But those situations are rare, and I need to revisit
those and send a v2; so I don't see a problem with this one going in
during the meantime as-is.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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